1. Field of the Invention
The present invention relates to an exhaust gas purification catalyst, an exhaust gas purification apparatus using the same and an exhaust gas purification method, and in more detail, the present invention relates to an exhaust gas purification catalyst, which is suitable as a Three Way Catalyst for efficiently purifying carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) contained in exhaust gas discharged from a gasoline automobile, an exhaust gas purification apparatus using the same and an exhaust gas purification method.
2. Description of the Prior Art
Exhaust gas discharged from an internal combustion engine of an automobile or the like, or a combustion engine such as a boiler, contains harmful substances such as hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), and various exhaust gas purification technologies for purifying these have been proposed. As one of these, there is an exhaust gas purification technology for purifying harmful components in exhaust gas, by installment of a catalyst in an exhaust gas passage.
In a catalyst apparatus for purifying exhaust gas discharged from an internal combustion engine of an automobile or the like, various catalysts have been used depending on objects thereof. As a major catalyst component thereof, there is a platinum-group metal, and usually it is supported onto a refractory inorganic oxide with large surface area such as activated alumina, in a highly dispersed state (refer to Patent Literature 1).
As for the platinum-group metal, which is the catalyst component, platinum (Pt), palladium (Pd) and rhodium (Rh) have been known, which have been widely used as the exhaust gas purification catalyst in an internal combustion engine such as an automobile. In many cases, in the above-described Three Way Catalyst (TWC: Three Way Catalyst), a catalytically activated species superior in oxidation activity, such as Pt and Pd, and Rh, which is superior in purification activity of NOx, are used in combination. In recent years, regulations on harmful substances, in particular NOx, contained in exhaust gas, have been increasingly tightened. Therefore, it is necessary to efficiently use Rh, which is superior in purification activity of NOx, however, because output of Rh is low and Rh price is expensive, market price thereof has been rising. Therefore, it is desired to reduce use amount of Rh, as a catalytically activated species, in view of resource protection or a cost aspect.
In order to reduce use amount of Rh, it is preferable that Rh is formulated more at the front layer side of the catalyst, so that Rh is arranged at a position easy to contact with exhaust gas (refer to Patent Literature 8). In this Patent Literature 8, there has been described an exhaust gas purification catalyst wherein ceria-type solid state oxide powder, which supports Rh, is contained in the upper layer, and at least either of Pt and Pd is supported onto hollow-state oxide powder in the lower layer.
In addition, in order to enhance further purification performance in the exhaust gas purification catalyst, various co-catalyst components other than the platinum-group metal are added. As such co-catalyst components, an oxygen storage component, (OSC: Oxygen Storage Component), an alkaline earth metal, zirconium oxide, zeolite and the like are known.
Among them, the OSC is one which occludes and discharges oxygen in exhaust gas, and cerium oxide is known as an example. Cerium oxide occludes oxygen as CeO2 when oxygen concentration is high in exhaust gas, and discharges oxygen as Ce2O3 when oxygen concentration is low. Oxygen discharged is active oxygen to promote purification of HC and CO by being utilized in oxidation action by Pt or Pd. In addition, the OSC also acts in buffering the change of oxygen concentration in exhaust gas, by occlusion and discharging of oxygen. This action enhances purification performance of exhaust gas in the TWC.
The TWC is one performing oxidation and reduction by one catalyst, and has a range of exhaust gas components suitable for purification, in view of designing. This range depends on air/fuel ratio in many cases. Such a range is called a window, in many cases, exhaust gas after combustion at the vicinity of theoretical air/fuel ratio is set in the window region. By buffering the change of oxygen concentration in exhaust gas, this window region can be maintained for a long period of time, and purification of exhaust gas is performed efficiently. It is said to give influence on purification performance of NOx, in particular, by Rh.
As such OSC, pure cerium oxide may be enough, however, it is used as a composite oxide with zirconium (refer to Patent Literature 2). The cerium-zirconium composite oxide has high heat resistance and also high oxygen storing speed. It is because the cerium-zirconium composite oxide has a stable crystal structure, does not inhibit the action of cerium oxide, which is a major OSC component, and thus acts as the OSC as far as the inside part of a particle.
In addition, it is desirable generally that such OSC has a high specific surface area value. The high specific surface area value means large active surface and is said that high activity as the OSC is also exhibited.
In purification of NOx by Rh, a steam reforming reaction is promoted as follows by the Rh component.HC+H2O - - - →COx+H2  (1)H2+NOx - - - →N2+H2O  (2)
And, use of a zirconium oxide together with the Rh component promotes the steam reforming reaction (refer to Patent Literature 3).
As the co-catalyst component, other than this, an alkaline earth metal such as a Ba component is known (refer to Patent Literature 4). The Ba component temporarily occludes NO contained in exhaust gas as Ba(NO3)2, and reduces and purifies NO occluded to N2 by a reducing component contained in exhaust gas.
In general, when fuel to be supplied to an engine is scarce, quantity of air is rich and combustion temperature is high, NO is generated in a large quantity. The Ba component temporarily absorbs NOx thus generated.
NOx absorbed by the Ba component is discharged from the Ba component, when NOx concentration becomes low and CO concentration becomes high in exhaust gas. This is caused by a reaction of the above Ba(NO3)2 with CO to be converted to BaCO3, and is also said chemical equilibrium. NOx discharged from the Ba component reacts with the reducing component at the surface of the Rh component as described above, and thus is reduced and purified.
As such a co-catalyst component, two or more can be used in combination, for example, the TWC using the Ba component and cerium oxide is known (refer to Patent Literature 5). However, purification performance may be lowered depending on combination of catalyst materials, for example, it has been reported that presence of the Rh component and the Ba component in the same composition reduces purification performance of NOx (refer to Patent Literature 6). The reason for this is not certain, however, it is considered to be caused by interference of purification action of NOx in the Rh component, because the alkaline earth metal component has occlusion action of NOx, and by making an alloy of the Ba component and the Rh component.
In this way, because there are various combinations of the catalyst components, and the catalyst components mutually take complicated reaction routes by correlated action, combinations of the catalyst components for exerting best purification performance have been searched by investigating these comprehensively.
It should be noted that the exhaust gas purification catalyst is enough to be arrange only one in an exhaust gas passage, however, there may be the case where two or more are arranged. Because catalyst surface area is increased by this, purification performance of exhaust gas enhances. However, as described above, because there is the window region in designing in the exhaust gas purification catalyst such as the TWC, there may be the case where desired purification performance cannot be obtained only by simple arrangement of a plurality of catalysts. It is because components of the exhaust gas, which passed through a catalyst of the former stage, has composition different from that of exhaust gas just after discharged from an engine, and a catalyst of the latter stage is necessary to be designed so as to have the composition of exhaust gas thus changed, as the window region.
Therefore, the present applicant have proposed a method for obtaining desired purification performance by using a catalyst system where two catalysts having a platinum-group metal and a specified oxygen storage component (OSC) as catalyst components are arranged in an exhaust gas passage (refer to Patent Literature 7).
In general, because the exhaust gas purification catalyst enhances purification activity of exhaust gas, when temperature is raised to a certain degree, in the case where an engine is started from a completely cooled state, sufficient purification performance cannot be exerted, in some cases, till the exhaust gas purification catalyst is warmed. In the case of purifying exhaust gas from an automobile by such catalyst technology, such performance has been required that is capable of purifying harmful components in exhaust gas in higher efficiency as compared with conventional technology, in a wider temperature region covering from start-up of an engine in a cooled state to a state where the exhaust gas is warmed and catalyst temperature is raised.
In addition, in recent years, regulation values for, in particular, NOx, have been increasingly tightened, and needs for the exhaust gas purification catalyst superior in purification performance of NOx has been increasing in the TWC too.    Patent Literature 1: JP-A-5-237390    Patent Literature 2: JP-B-06-75675    Patent Literature 3: JP-A1-2000/027508, page 14    Patent Literature 4: JP-A-2007-319768, paragraph 0003    Patent Literature 5: JP-A-03-106446    Patent Literature 6: JP-A-2002-326033, paragraph 0013    Patent Literature 7: JP-A-2008-68225    Patent Literature 8: JP-A-2006-159159, claim 3